The most progressive US president in a generation comes to the most important international meeting since the Second World War and delivers a speech so devoid of substance that he might as well have made it on speaker-phone from a beach in Hawaii. His aides argue in private that he had no choice, such is the opposition on Capitol Hill to any action that could challenge the dominance of fossil fuels in American life. And so the nation that put a man on the Moon can't summon the collective will to protect men and women back here on Earth from the consequences of an economic model and lifestyle choice that has taken on the mantle of a religion. Then a Chinese premier who is in the process of converting his Communist nation to that new faith (high-carbon consumer capitalism) takes such umbrage at Barack Obama's speech that he refuses to meet – sulking in his hotel room, as if this were a teenager's house party instead of a final effort to stave off the breakdown of our biosphere.

Late in the evening, the two men meet and cobble together a collection of paragraphs that they call a "deal", although in reality it has all the meaning and authority of a bus ticket, not that it stops them signing it with great solemnity.

Obama's team then briefs the travelling White House press pack – most of whom, it seems, understand about as much about global-climate politics as our own lobby hacks know about baseball. Before we know it, The New York Times and CNN are declaring the birth of a "meaningful" accord.

Meanwhile, a friend on an African delegation emails to say that he and many fellow members of the G77 bloc of developing countries are streaming into the corridors after a long discussion about the perilous state of the talks, only to see Obama on the television announcing that the world has a deal.

It's the first they've heard about it, and a few minutes later, as they examine the text, they realise very quickly that it effectively condemns their continent to a century of devastating temperature rises.

By now, the European leaders – who know this thing is a farce but have to present it to their publics as progress – have their aides phoning the directors of civil society organisations spinning that the talks have been a success.

A success? This deal crosses so many of the red lines laid out by Europe before this summit started that there are scarlet skid marks across the Bella Centre, and one honest European diplomat tells us this is a "shitty, shitty deal". Quite so.

This "deal" is beyond bad. It contains no legally binding targets and no indication of when or how they will come about. There is not even a declaration that the world will aim to keep global temperature rises below C. Instead, leaders merely recognise the science behind that vital threshold, as if that were enough to prevent us crossing it.

The only part of this deal that anyone sane came close to welcoming was the $100bn global climate fund, but it's now apparent that even this is largely made up of existing budgets, with no indication of how new money will be raised and distributed so that poorer countries can go green and adapt to climate change.

I know our politicians feel they have to smile and claim success; they feel that's the only way to keep this train on the tracks. But we've passed that point – we need to go back to first principles now. We have to admit to ourselves the scale of the problem and recognise that at its core this carbon crisis is, in fact, a political crisis.

Not all of our politicians deserve the opprobrium of a dismayed world. Ed Miliband fought hard, on no sleep, for a better outcome; while Brazil's President Luiz Inacio Lula da Silva offered to financially assist other developing countries to cope with climate change, and put a relatively bold carbon target on the table. But the EU didn't move on its own commitment (one so weak we'd actually have to work hard not to meet it), while the United States offered nothing and China stood firm.

Before the talks began, I was of the opinion that we would know Copenhagen was a success only when plans for new coal-fired power stations across the developed world were dropped. If the giant utilities saw in the outcome of Copenhagen an unmistakable sign that governments were now determined to act, and that coal plants this century would be too expensive to run under the regime agreed at this meeting, then this summit would have succeeded.

Instead, as details of the agreement emerged last night, we received reports of Japanese opposition MPs popping champagne corks as they savoured the possible collapse of their new government's carbon targets.

It's not just that we didn't get to where we needed to be, we've actually ceded huge amounts of ground. There is nothing in this deal – nothing – that would persuade an energy utility that the era of dirty coal is over. And the implications for humanity of that simple fact are profound.

I know we environmentalists are partial to hyperbole. We use language as a bludgeon to direct attention to the crisis we are facing, and you will hear much more of it in the coming days and weeks. But, really, it is no exaggeration to describe the outcome of Copenhagen as a historic failure that will live in infamy.

In a single day, in a single space, a spectacle was played out in front of a disbelieving audience of people who have read and understood the stark warnings of humanity's greatest scientific minds. And what they witnessed was nothing less than the very worst instincts of our species articulated by the most powerful men who ever lived.